High gain amplifier circuits



Nov. 4, 1941. G. MOUNTJOY 2,261,401

` HIGH GAIN AMPLIFIER CIRCUITS Filed Aug. l, 1940 Cobrar/l' j -7 ZMZe/c/'ar 14 J/'g/m/ Ol/'CQ l l l' 7g- M /Ve'wor/ ,CM L+@ n ecr .AL/DI i l/rmr' 30 FMoz/Rce I a7 [E Z5 Snncr'lfcn' Gerrard Mounj og attorney Patented Nov. 4, 1941 2,261,401 HIGH GAIN AMPLIFIER. CIRCUITS Garrard Mountjoy, Manhasset, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application August 1, 1940, seria1N0.349,047

1 Claim.

My present invention relates to high frequency amplifier circuits of high gain, and more particularly to a novel method of, and means for, providing high gain intermediate frequency amplifier circuits.

It has been demonstrated in the past ythat tuned high frequency amplifier circuits have a definite stability limit which is dependent upon the loading of the input and output networks. In other words, it has previously been shown that there is a limit imposed upon. the designer of a tuned high frequency amplifier circuit, such as one operating in the intermediate frequency range for example, and that the input and output networks of an amplifier circuit cannot have a predetermined upper limit of impedance exceeded if regenerative feed-back through the control grid to plate capacity of the amplifier tube is to be prevented. It has, therefore, been necessary in the past to utilize cascaded amplifier circuits in order to provide the necessary overall gain without running into instability. Even in the case of cascaded tunedhigh frequency amplifier circuits, it has been necessary carefully to design the input and output networks of the various stages so as to prevent instability due to regenerative feed-back from stage to stage.

It may, therefore, be stated that it is one of the main objects of my present invention to provide a tuned high frequency amplifier circuit which utilizes terminal input and output networks constructed in such a manner that high gain can be secured from the amplifier circuit with a relatively less number of stages than has e been deemed necessary in the past and without running into instability.

Another important object of this invention is to provide a high gain intermediate frequency amplifier circuit wherein the terminal impedances of the circuit are constructed essentially to provide the gain therefor without permitting regenerative feed-back other than through the signal control grid to plate capacity of the amplifier tubes.

Another object of this'invention is to provide a high frequency amplifier tube whose high frequency input circuit and high frequency output circuit are sufficiently low in magnitude of impedances to prevent regenerative feed-back between the plate and input electrode of the tube, and the input and output circuits each being coupled to a similarly tuned high frequency circuit of substantially high impedance thereby to provide a high gain for the amplifier circuit, and

each of said similarly tuned circuits being in turn coupled to networks whose operating frequency ranges do not include the frequency of said amplifier tuned circuits.

Still another object of this invention is to provide in a superheterodyne receiver, and between the converter stage and the second detector stage, at least one intermediate frequency amplifier tube which has grid and plate circuits of sufciently low impedance value to prevent regenerative feed-back from the plate to the signal grid, the converter being coupled to the amplifier input circuit by an intermediate frequency circuit of relatively high impedance value, and the amplifier output circuit being coupled to the second detector through an intermediate frequency circuit of relatively high impedance value whereby a relatively high overall gain is produced for the amplifier circuit. i

Still other objects of this invention are to improve generally the efficiency and simplicity of high gain intermediate frequency amplifier cir-` cuits, and more especially to provide such circuits which are not only efficient and stable in operation, but are, additionally, economically manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claim; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a superheterodyne receiving system embodying the invention, and

Fig. 2 illustrates, in a schematic manner, the embodiment of the invention in a receiver of frequency modulated carrier waves.

` Referring now to the accompanying drawing, wherein like reference characters of the different figures designate similar circuit elements, the receiving arrangement shown in Fig. 1 is of the well-known superheterodyne type. The invention will be explained in this type of receiver because` it is substantially universally employed in the standard broadcast range of 550-1700 kilocycles (kc). However, it is to be clearly understood that the invention is not limited to utilization in connection with the intermediate frequency (I. F.) amplifier of a superheterodyne receiver, since the invention may be employed in any desired frequency range employed in radio communication. Assuming, however, for the purpose of this application that the numeral I is the tunable signal input circuit of a superheterodyne receiver, the latter will be coupled to any desired type of modulated signal carrier energy collector. While the input circuit I is shown as being connected directly to the converter stage, it is to be understood that one or more stages of radio frequency amplification may be employed prior to the converter.

The numeral 2 denotes the usual tunable oscillator tank circuit which is employed in the converter network. The converter itself may be a combined local oscillator-first detector network which employs a pentagrid converter tube of the 6A7 or 6A8 types. rIhose skilled in the art are fully acquainted with the manner of utilizing such tubes in a converter stage. Of course, separate rst detector and local oscillator tubes may be utilized. However, in either case the numeral 3 designates the mechanical unicontrol tuner, shown as a dotted line, which simultaneously adjusts the rotors of the variable tuning condensers of circuits I and 2. The tank circuit 2 will, of course, be varied through a frequency range which constantly differs from the frequency range of circuit I by the value of the operating I. F. The latter may be chosen from a range of 75 kc. to 465 kc. For the purposes of illustration, let it be assumed that the operating I. F. Value is 465 kc.

The numeral 4 denotes the I. F. output circuit of the converter, and the circuit 4 comprises a coil 5 and a shunt condenser 6. The circuit 4 is resonated to the operating I. F. value, and coil 5 is magnetically coupled to the input coil 'I of the following resonant circuit which utilizes the shunt condenser 8 to tune coil 'I to the operating I. F. value. The I. F. amplier tube 9 may be of any desired type, and is illustratively shown as of the screen grid type. The numeral I0 designates the usual by-passed self-bias resistor which provides the operating negative bias for the control grid II of tube 9.

In the plate circuit of tube 9 there is included the intermediate frequency output circuit which comprises coil I2 and shunt condenser I3, it being understood that the circuit I2-I3 is fixedly resonated to the operating I. F. value. In

order to simplify the description but one stage of I. F. amplification is illustrated; it is to be clearly understood that a plurality of I. F. arnplier circuits of the type shown in Fig. 1 may be utilized in cascade, if desired.

The second detector, or demodulator, of the receiver comprises a diode I4 whose anode is connected to the high potential end of the input coil I4, the latter being shunted by the condenser I5. The circuit I 4-I5 is fixedly resonated to the operating I. F. value, and coils I2 and I4 are magnetically coupled. The load resistor I6 is connected between the low potential end of coil I4 and the grounded cathode of diode I4', the load resistor I6 being appropriately by-passed by an I. F. by-pass condenser. The .f1

modulation voltage component, and this will be audio frequency in the case of speech or music broadcasting, developed across resistor I6 may be impressed upon a slidable tap II for utilization in a subsequent audio frequency network. The latter, of course, may comprise one or more audio frequency amplifiers followed by any desired type of reproducer. The direct current voltage component developed across resistor I6 may be employed for automatic volume control I. F. amplifier stage.

(AVC) after appropriate filtering of the pulsation voltage components. The AVC bias may be applied to the signal grids of any of the earlier transmission tubes preceding the detector diode I4.

In past practice it has been found that the plate to grid capacity of the I. F. amplier 9, and this capacity is designated by numeral 20 in dotted lines in Fig. 1, has made it necessary that the grid and plate circuits of the amplifier` tube be maintained in impedance value below a predetermined upper limit. In other Words, it has been demonstrated that if the networks 1-8 and I3I2 were chosen to have impedances sufficiently large to secure a relatively high gain from the amplifier stage, and if these impedance values were above predetermined permissible values, then regenerative feed-back through the inherent plate to grid capacity 20 would occur with subsequent instability of the amplifier. Such instability, of course, manifests itself by squeals, howls, etc. Hence, it has been the practice to design the grid and plate loads of the I. F. ampliiier stage below the said upper limit, and then to utilize other I. F. stages in cascade. However, even this precaution has not solved the problem because interstage regenerative feed-back occurs.

Now, according to my present system, there is provided the desirable high overall gain for the I. F. amplifier without creating the conditions for instability, and this is accomplished in a highly economical and compact manner. In accordance with my invention the primary circuit 6-5 which feeds the I. F. energy into the signal grid circuit 'I-B is given a relatively high impedance value. Similarly, the secondary I. F. circuit I 4-I5, which is reactively coupled to the plate circuit I3-I2 of the I. F. amplier, is given a relatively high impedance value. Both circuits 6 5 and I4--I5 are given sufficiently high impedance values to produce that amount of high overall gain which the set designer normally desires, but is prevented from securing because of the desire to avoid regenerative feed-back in the t is impossible to set up regenerative feed-back in the network feeding the circuit 4, or in the network following circuit I4--I5, since neither of these networks includes the operating I. F. value of these terminals 4 and I4-I5.

This is readily seen from the fact that the converter has the signal and local oscillation frequencies applied thereto as input frequencies, and both of these frequencies are far in excess of the operating I. F. value. In the case of circuit I4-I5 the second detector network provides the modulation voltage which is relatively far below the I. F. value. It will then be seen that the only regeneration that can occur between the converter and the second detector is through the inherent capacity 20. However, regeneration through that capacity path is effectively prevented because the coils 1 and I2 have been designed in the first place to reduce any regenerative feed-back through capacity 20. Furthermore, it can be shown that reducing the magnitude of coil l or I2 has only a square root effect on the gain of the stage. On the other hand, making the coils 5 and I4 relatively large in magnitude has no effect on producing regeneration, and produces a square root effect on gain. It can be shown that the overall gain of the I. F. ampliiier stage is proportional to a constant times w/LizLm and a constant times \/L5L7. By making L7 and L12 small, a condition of stability may be reached. L7 and L12 are made as large as possible without producing too much regeneration. L5 and L14 do not affect regeneration but do affect gain, and are made relatively larger. The regeneration through the I. F. amplifier is proportional to LvLiz.

It will now be seen that it is possible to provide as many stages of amplification as is desired in cascade without fear of regenerative feed-back occurring between stages. It is merely necessary in the case of each stage to construct the grid and plate circuits of the amplifier tube to be of suiciently low impedance values so as to prevent regenerative feed-back between the plate and signal grid of the stage while providing tuned impedances in association with the input and output of the amplifier of such high magnitude that the desirable high gain is secured. The high impedance circuits will, of course, be the terminal circuits of the amplifier network, and should look f into networks of frequency ranges substantially diiferent from, or not including, the operating frequency of the amplifier network. By means of the present invention, and considering the case of Fig. l, it is possible to increase the stage gain as much as live times or more while maintaining stability. By way of illustration, C6=5 micromicrofarads (mmf.); Cs=100 mmf., C13=100 mmf., and C15=5 mmf.

In Fig. 2 there is shown, in schematic manner, the limiter stage 20 of a frequency modulated carrier wave receiver. Those skilled in the art are fully aware at the present time of the manner of construction of a frequency modulated (FM) wave receiver, and it is not believed necessary to describe the various networks of such a receiver in detail. Let it be assumed that with the FM band in the 42-50 megacycle (mc.) range, the received FM waves are converted to an I. F. value of 4.3 mc. The latter value is, of course, in the case of F. M. waves the center, or mean, frequency of the FM signals. In an FM receiver it is highly desirable to deliver to the limiter input network a high signal voltage level. The function of the limiter is to eliminate any amplitude variation in the FM signals, and to feed to the following FM detector 2| signals of substantially constant amplitude. The present invention is readily adapted to the networks be- Y tween the converter and limiter input circuit 22, and it is only necessary for the purposes of this application to point out that the resonant input circuit 22 of the limiter would be designed in a manner similar to that described in connection with circuit I4-l 5 in Fig. 1.

That is, the circuit 22, which is tuned to the center frequency of 4.3 mc., is given a relatively high impedance value. The preceding I. F. tuned network |3--I2 is, as stated in connection with Fig. 1, maintained substantially low in impedance value so as to prevent its I. F. amplifier tube 9 from regeneratively feeding back I. F. energy between the plate and signal grid. The ouput electrode of the limiter tube is preferably coupled to the FM detector 2| through a low impedance resistance-capacity coupling 3B. It will be seen that the terminal circuit 22 looks into a network which is resonated to the I. F. value. In this connection it is desirable to design the detector 2l in the manner of the well known back to back type. Those skilled in the art are well acquainted with this type of detector, and it need only be pointed out that in such detector a pair of rectiers are arranged to have their output circuits in phase opposition, whereas their input circuits are oppositely mistuned from the FM center frequency by equal frequency magnitudes.

While I have indicated and described a system for carrying my invention into'effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, b-ut that many modifications may be made vwithout departing from the scope of my invention,.as set forth in the appended claim.

What is claimed is:

In combination, in a superheterodyne receiver of the frequency modulated type including a converter network, a limiter stage and a frequency modulation detector network, an intermediate frequency amplifier network including at least one tube having input and output circuits each tuned to the operating intermediate frequency value, said input and output circuits being of suciently low impedance value to prevent regenerative feed-back between said output and input circuits, a resonant circuit tuned to said intermediate frequency coupling said converter to said amplier input circuit, a resonant circuit tuned to said intermediate frequency coupling said amplifier output circuit to said limiter stage, both of said resonant circuits being of substantially higher impedance value than said amplifier input and output circuits, and a ylow impedance resistance-capacity network coupling the output of the limiter stage to the input of the frequency modulation detector.

GARRARD MOUNTJOY. 

